As the number of channels available to television viewers has increased, along with the diversity of the programming content available on such channels, it has become increasingly challenging for service providers, such as cable television operators and digital satellite service operators, to offer packages of channels and programs that satisfy the majority of the television viewing population. The development of packages that may be offered to customers is generally a marketing function. Generally, a service provider desires to offer packages of various sizes, from a single program to all the programs, and various combinations in between.
The service provider typically broadcasts the television programs from a transmitter, often referred to as the "head-end," to a large population of customers. Each customer is typically entitled only to a subset of the received programming, associated with purchased packages. In a wireless broadcast environment, for example, the transmitted programming can be received by anyone with an appropriate receiver, such as an antenna or a satellite dish. Thus, in order to restrict access to a transmitted program to authorized customers who have purchased the required package, the service provider typically encrypts the transmitted programs and provides the customer with a set-top terminal (STT) containing one or more decryption keys which may be utilized to decrypt programs that a customer is entitled to. In this manner, the set-top terminal receives encrypted transmissions and decrypts the programs that the customer is entitled to, but nothing else.
In order to minimize piracy of the highly sensitive information stored in the set-top terminals, including the stored decryption keys, the set-top terminals typically contain a secure processor and secure memory, typically having a capacity on the order of a few kilobits, to store the decryption keys. The secure memory is generally non-volatile, and tamper-resistant. In addition, the secure memory is preferably writable, so that the keys may be reprogrammed as desired, for example, for each billing period. The limited secure memory capacity of conventional set-top terminals limits the number of keys that may be stored and thereby limits the number of packages which may be offered by a service provider. It is noted that the number of programs typically broadcast by a service provider during a monthly billing period can be on the order of 200,000.
In one variation, conventional set-top terminals contain a bit vector having a bit entry corresponding to each package of programs offered by the service provider. Typically, each package corresponds to one television channel. If a particular customer is entitled to a package, the corresponding bit entry in the bit vector stored in the set-top terminal is set to one ("1"). Thereafter, all programs transmitted by the service provider are encrypted with a single key. Upon receipt of a given program, the set-top terminal accesses the bit vector to determine if the corresponding bit entry has been set. If the bit entry has been set, the set-top terminal utilizes a single stored decryption key to decrypt the program.
While, in theory, flexibility is achieved in the bit vector scheme by providing a bit entry for each program, the length of the bit vector would be impractical in a system transmitting many programs in a single billing period. In addition, access control in such a system is provided exclusively by the entries in the bit vector and is not cryptographic. Thus, if a customer is able to overwrite the bit vector, and set all bits to one ("1"), then the customer obtains access to all programs.
In a further variation, programs are divided into packages, and all programs in a given package are encrypted using the same key. Again, each package typically corresponds to one television channel. The set-top terminal stores a decryption key for each package the customer is entitled to. Thus, if a program is to be included in a plurality of packages, then the program must be retransmitted for each associated package, with each transmission encrypted with the encryption key corresponding to the particular package. Although the access control is cryptographic, the overhead associated with retransmitting a given program a number of times discourages service providers from placing the same program in a number of packages and thereby limits flexibility in designing packages of programs.
While such previous systems for encrypting and transmitting programming content have been relatively successful in restricting access to authorized customers, they do not permit a service provider, such as a television network, to offer many different packages containing various numbers of programs to customers, without exceeding the limited secure memory capacity of the set-top terminal. As apparent from the above-described deficiencies with conventional systems for transmitting encrypted programming content, a need exists for a system for transmitting a program encrypted with a unique key, together with the unique key used to encrypt the program. A further need exists for a system that permits a service provider to include a program in a plurality of packages, without requiring the service provider to retransmit the program for each package. Yet another need exists for an access control system that overcomes the secure memory limitations of the set-top terminal without significantly increasing the overhead associated with the transmitted programming content.